New GlowWorm 30CXI combi boiler with pressure drop problems since installation late 2006.

Day one ....New Diverter Valve fitted before it would run.
Pressure dropped slowly in first few months. No loss with CH off.

Under concrete floor system isolated and checked with 5 pressure (Rothenburger pump) 24 hours - No loss shown.
New pressure relief valve and pressure sensor fitted by Glow Worm.
Glow Worm replaced pressure vessel as holding no air when checked.
Checked system by isolating boiler from CH for 17 hours - no loss in pressure when turned back on.
Pressure continues to drop to switch off within 3 days -after setting to 1.5
Is there any way of being 100% sure of no leak in piping - Corgi reg CH Plumber and Glow Worm so far no answer.

Does the pressure rise much when the central heating is on, if it rises then gets discharged to outside via the pressure relief valve - this could be why the pressure is dropping. The fault could be with the expansion vessel or mains cold/DHW mixing with heating side.

Keep a close eye on the pressure relief discharge pipe - if there's no drips or any water at all. Then there must be a leak somewhere, check for leaks on :-

You could attach something to the pressure relief dicharge pipe to catch any water or to give a sign it is passing. As long as you don't block it from discharging - put a balloon over the end, if it fills or comes off then there's the problem - that way your not restricting the flow as the balloon will just come off or burst.

Many thanks
We have already checked all you have suggested. We may give the balloon a try to check air loss - but have already used a plastic bottle to check any water leak from relief valve. As a pressure of 5 was put through the ch piping it appears there is no leak there.
Expansion vessel recently replaced but no change.
Pressure does not go up a great deal when heating is on, and as soon as the heating is off the pressure goes down by about 0.3 in approx 3 hours. Then at 0.6 shuts off. Glow Worm are considering replacing.